The present invention is directed, in general, to wireless communications systems and, more specifically, to a system and method for tuning a narrowband cavity filter in an RF transmit path of a CDMA transmitter.
In order to increase the number of subscribers that can be serviced in a single wireless network, frequency reuse is maximized by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. Accordingly, the greater number of base transceiver stations increases infrastructure costs. To offset this increased cost, wireless service providers continually implement any improvements that may reduce equipment costs, maintenance and repair costs, and operating costs, or that may increase service quality and reliability, and the number of subscribers that the cellular system can service.
One opportunity for obtaining cost improvement in base transceiver stations involves reducing filter costs on the output of a power amplifier in a base station transmitter of a wireless network. Prior art transmitters use either fixed narrow bandwidth filters or fixed wide bandwidth filters. Such filters are placed on the output of a power amplifier in a CDMA transmitter to reject adjacent channel products (ACP) that are generated by the power amplifier, to reject transmitter noise that falls in the receive band, and to meet operating standards for inband and out-of-band requirements. Fixed wide bandwidth filters have the disadvantage that they cannot efficiently reject adjacent channel products. Although the fixed narrow bandwidth filters can effectively reject adjacent channel products, the fixed narrow bandwidth filters are channel specific and must be individually selected and installed in each transmitter to meet specific customer requirements. For example, in one transmit channel within the 1930 Hz to 1990 MHz range of an IS-95 compliant CDMA wireless network, the output filter requires approximately fifty (50) different individual output filters to configure a particular base transceiver station for operation.
Therefore, there is a need for an improved base transceiver station having less expensive filters and having a lower level of power consumption. In particular, there is a need to reduce the number of filters that are used on the output of the power amplifier in a base transceiver station. More particularly, there is a need to reduce the number of filters that are used on the output of the power amplifier in a base transceiver station while still providing coverage for the full dynamic range of the RF transmitter.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a system and method for tuning a tunable narrowband cavity filter in a base station transmitter of a wireless network. In an advantageous embodiment of the present invention, the radio frequency (RF) transmitter comprises: 1) modulation circuitry capable of modulating an input baseband signal to produce a first RF signal; 2) amplification circuitry capable of amplifying the first RF signal; 3) a tunable narrowband cavity filter capable of filtering the amplified first RF signal and transmitting the filtered and amplified first RF signal to an antenna coupled to an output of the tunable narrowband cavity filter; and 4) a system for tuning the tunable narrowband cavity filter. The system for tuning the tunable narrowband cavity filter comprises: a) a filter calibration signal injection circuit capable of combining an input calibration signal with the input baseband signal; and b) a filter calibration circuit coupled to the output of the tunable narrowband cavity filter capable of detecting an RF output calibration signal corresponding to the input calibration signal, determining a signal level of the RF output calibration signal, and adjusting a center frequency of a pass band of the tunable narrowband cavity filter in response to the signal level determination.
According to one embodiment of the present invention, the filter calibration signal injection circuit comprises a signal source capable of generating the input calibration signal; a first summation circuit coupled to the signal source capable of combining the input calibration signal with an in-phase baseband signal to produce a first summed signal; and a second summation circuit coupled to the signal source capable of combining the input calibration signal with a quadrature baseband signal to produce a second summed signal.
According to another embodiment of the present invention, the modulation circuitry combines and modulates the first and second summed signals to produce the first RF signal.
According to still another embodiment of the present invention, a level of the input calibration signal is at least 40 dB lower than a level of the in-phase baseband signal and wherein the level of the input calibration signal is at least 40 dB lower than a level of the quadrature baseband signal.
According to yet another embodiment of the present invention, the filter calibration circuit comprises an RF mixer capable of receiving the RF output calibration signal from the tunable narrowband cavity filter and demodulating the. RF output calibration signal to produce a demodulated baseband calibration signal; and a filter calibration controller capable of receiving the demodulated baseband calibration signal, determining a signal level of the demodulated baseband calibration signal, and adjusting the center frequency of the pass band of the tunable narrowband cavity filter according to the signal level of the demodulated baseband calibration signal.
According to a further embodiment of the present invention, the RF output calibration signal comprises an upper RF calibration signal disposed at approximately X Hz above a center frequency of the filtered and amplified first RF signal and a lower RF calibration signal disposed at approximately X Hz below the center frequency of the filtered and amplified first RF signal.
According to a still further embodiment of the present invention, the RF mixer demodulates the upper and lower RF calibration signals to produce the demodulated baseband calibration signal, wherein the signal level of the demodulated baseband calibration signal is proportional to a combined signal level of the upper and lower RF calibration signals.
According to a yet further embodiment of the present invention, the filter calibration controller adjusts the center frequency of the pass band of the tunable narrowband cavity filter to minimize the signal level of the demodulated baseband calibration signal.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.